Introduction: Important Calculations in Electronics

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This Instructable is intend to list out some of the important calculations in electronics engineers/makers need to be aware of. Quite frankly there are lots of formulas that can fit into this category. So i have limited this Instructable to basic formulas only.

For most of the formulas listed I have also added link to online calculators that can help you to perform these calculations with ease when it becomes cumbersome and time consuming.

Step 1: Battery Life Calculator

When powering projects using batteries, it is essential that we know the expected duration a battery can power your circuit/ device. This is important to extend the life of battery and prevent unexpected failure of your project. There are two important formula associated with this.

Maximum duration a battery can power a load

Battery life = Battery capacity ( mAh or Ah ) / Load current ( mA or A )

Rate at which load draws current from the battery

Rate of discharge C = Load current ( mA or A ) / Battery capacity ( mAh or Ah )

Rate of discharge is an important parameter which decides how much current a circuit can safely draw from a battery. This is usually marked in the battery or will be given in its datasheet.


Battery capacity = 2000mAh , Load current = 500mA

Battery Life = 2000mAh / 500mA = 4 hours

Rate of discharge C = 500mA/2000mAh = 0.25 C

Here is a Battery life online calculator.

Step 2: Linear Regulator Power Dissipation

Linear regulators are used when we need a fixed voltage to power a circuit or device. Some of the popular Linear voltage regulators are 78xx series ( 7805, 7809, 7812 and so on ). These linear regulator work by dropping the input voltage and gives steady output voltage in output. The power dissipation in these linear regulators are often overlooked. Knowing the power dissipated is quite important so designers can use heatsinks to compensate for high power dissipation. This can be calculated using the below formula

Power dissipation is given by the formula

PD = ( VIN - VOUT ) x IOUT

To calculate the output current

IOUT = PD / ( VIN - VOUT )


Input voltage - 9V, Output voltage - 5V, Current output -1A Result


= ( 9 - 5 ) * 1

= 4Watts

Online calculator for linear regulator power dissipation.

Step 3: Voltage Divider Calculator

Voltage dividers are used to divide the incoming voltages to desired voltage levels. This is highly useful to produce reference voltages in circuits. Voltage divider is generally built using at least two resistors. Learn more about how voltage dividers work. The formula used with voltage dividers are

To determine output voltage Vout = ( R2 x Vin ) / ( R1 + R2 )

To determine R2 R2 = ( Vout x R1 ) / ( Vin - Vout )

To determine R1 R1 = ( ( Vin - Vout ) R2 ) / Vout

To determine input voltage Vin = ( Vout x (R1 + R2) ) / R2


Vin=12 V , R1=200k, R2=2k

Vout = ( R2 x Vin ) / ( R1 + R2 )

Vout = (2k x 12)/(200k+2k)


=0.12 V

Step 4: RC Timing Calculator

RC circuits are used to generate time delays in many circuits. This is due to the action of resistor influencing the charging current that flows to capacitor. The greater the resistance and capacitance, the more time it takes for capacitor to charge and this will be exhibited as delay. This can be calculated using the formula.

To determine Time in seconds

T = RC

To determine R

R = T / C

To determine C

C = T / R


R = 100K , C = 1uF

T = 100 x 1 x 10^-6

T = 0.1ms

Try this RC time constant online calculator.

Step 5: LED Resistor

LED's are quite common is electronic circuits. Also LEDs will be often used with current limiting series resistor to prevent excess current flow damage. This is the formula used to calculate the series resistor value used with LED

R = ( Vs - Vf ) / If


If you are using LED with Vf = 2.5V , If = 30mA and Input voltage Vs = 5V . Then resistor will be

R = ( 5 - 2.5V ) / 30mA

= 2.5V / 30mA

= 83Ohm

Step 6: Astable and Monostable Multivibrator Using IC 555

555 IC is a versatile chip which has wide array of applications. Right from generating square waves, modulation, time delays, device activation, 555 can do it all. Astable and Monostable are two commonly used modes when comes to 555.

Astable multivibrator - It produces square wave pulse as output with fixed frequency. This frequency is decided by resistors and capacitors used with it.

With given RA, RC and C values. Frequency and Duty cycle can be calculated using the below formula

Frequency = 1.44 / (( RA +2RB) C)

Duty cycle = ( RA + RB ) / ( RA + 2RB )

Using RA, RC and F values, Capacitance can be calculated using the below formula

Capacitor = 1.44 / (( RA + 2RB ) F )


Resistance RA = 10 kohm, Resistance RB = 15 kohm, Capacitance C = 100 microfarads

Frequency = 1.44 / ((RA+2RB)*c)

= 1.44 / ((10k+2*15k)*100*10^-6 )

= 1.44 / ((40k)*10^-4)

= 0.36 Hz

Duty cycle =(RA+RB)/(RA+2RB)


= (25k)/(40k)

=62.5 %

Monostable multivibrator -

In this mode IC 555 will produce high signal for a certain period of time when trigger input goes low. It is used to generate time delays.

With given R and C, we can calculate Time delay using the below formula

T = 1.1 x R x C

To determine R

R = T / ( C x 1.1 )

To determine C

C = T / ( 1.1 x R )


R=100k , C=10uF

T=1.1 x R x C

=1.1 x 100k x10uF


Here is online calculator for Astable multivibrator and Monostable multivibrator

Step 7: Resistance, Voltage, Current and Power ( RVCP )

We will start from the basics. If you are introduced to electronics you might have known the fact that Resistance, Voltage, Current and Power are all interrelated. Changing one of the above will alter other values. The formula for this calculation is

To determine voltage V = IR

To determine Current I = V / R

To determine resistance R = V / I

To calculate power P = VI


Let's consider the below values

R=50 V, I=32 mA

V = I x R

= 50 x 32 x 10^-3

= 1.6V

Then the power will be

P=V x I

=1.6 x 32 x10^-3


Here is an Online Ohms law calculator to calculate Resistance, Voltage, Current and Power.

I will update this Instructable with more formulas.

Leave your comments and suggestions below and help me to add more formulas to this Instructable.